Selective laser sintering is a solid freeform fabrication process which, in a layer-by-layer fashion, creates parts with precise dimensions requiring minimal, if any, machining. More specifically, the selective laser sintering process usually includes a laser directing a beam of electromagnetic radiation at a heat-fusible powder (hereinafter referred to as powder) causing the portion of powder struck by the beam to sinter. The beam scans back and forth across the powder and forms a sintered layer corresponding to a cross sectional portion of the part. When one layer is completely sintered, the laser is turned off, the sintered layer is lowered, a new layer of powder is spread over the previous, now sintered layer, and the new layer of powder is scanned by the beam. Scanning the new layer not only sinters the new layer but also causes the newly sintered layer to adhere to the previously sintered layer.
One problem with selective laser sintering, however, is that the beam often cannot penetrate the powder surface which inevitably sinters before the powder beneath the surface (hereinafter referred to as sub-surface powder) sinters. Consequently, the sub-surface powder must rely on thermal conductivity to adequately raise its temperature to the sintering level. When thermal conduction fails to transfer sufficient energy from the surface to the sub-surface powder, the sub-surface powder fails to sinter and adhere to the previously sintered layer.
The goal of sintering is to fuse particles while maintaining their integrity and exploiting their mechanical properties. For ceramic particles, it is important that the ceramic particles sinter without melting in order to prevent the destruction of the integrity of the particles. One attempt to ensure complete sintering of ceramic particles comprised continued contact by the beam on the powder surface, post sintering thereof, with the expectation that the excess energy would transfer to and sinter the sub-surface powder. This method failed to transfer sufficient energy to and sinter the sub-surface powder. Rather than sintering the sub-surface powder, the excess energy melted the powder surface, thereby adversely effecting the mechanical properties of the ceramic powder.
Sintering the sub-surface powder requires careful control of the powder temperature throughout the depth of the powder. Various methods of controlling the powder temperature have been employed. For example, U.S. Pat. No. 5,017,753, issued May 21, 1991, indicates that creating a downward flow of temperature-controlled air through the powder layer moderates the temperature differences within the powder layer. U.S. Pat. No. 5,352,405, issued Oct. 4, 1994, recognized that beam overlap of previously scanned regions created thermal inequities within these regions and attempted to dissipate these inequities by reducing the exposure time of the beam to the overlapping regions. U.S. Pat. No. 5,427,733, issued Jun. 27, 1995, addressed the problem of uneven sintering created from constant powered lasers by altering the beam intensity. The prior art, however, fails to teach a method for successfully sintering substantially all of a powder layer.
Thus, it would be desirable to devise a sintering process which sinters substantially all of the powder layer.